1. Field of the Invention
This invention relates to a method for removing carbon oxides from a hydrogen stream by treatment of the hydrogen stream in an aromatics saturation reactor.
2. Description of Related Art
In refinery processes it is desirable that a substantial portion of the crude oil or other petroleum feed stock to the refinery be converted to gasoline range materials. Gasoline comprises a hydrocarbon fraction generally having a boiling range of about 30 to about 430.degree. F. and a research octane number (RON) of at least about 90. A variety of refinery processes are used to increase the gasoline yield from crude oil charged to a refinery. Such processes include catalytic cracking, reforming, alkylation and the like. In the refining process naphthenic and paraffinic hydrocarbons are produced which are of a suitable boiling range for use as gasoline but which have an octane rating too low for use as gasoline. The octane rating of such hydrocarbons is typically increased by reforming. In the reforming process, the naphthene hydrocarbons and paraffin hydrocarbons are converted to aromatic hydrocarbons. As is well known to those skilled in the art, aromatic materials have a higher octane rating than similar boiling range paraffinic or naphthenic materials.
While such reforming processes are effective to produce higher octane rating materials, the materials so produced are aromatic and in recent years there have been requirements to reduce the aromatic component content of gasoline and other fuels. While reforming remains a valuable tool for increasing the octane rating of paraffinic hydrocarbons increased attention has been directed to other methods for increasing the octane rating of paraffinic hydrocarbons.
One such method is the use of isomerization. Isomerization of gasoline range paraffins is frequently used with paraffinic hydrocarbons containing from 5 to 6 carbon atoms. Such C.sub.5 /C.sub.6 streams are frequently subjected to benzene saturation and isomerization treatment to saturate benzene and other aromatics which may be present in minor amounts (herein benzene) with the C.sub.5 and C.sub.6 paraffins and convert straight chain C.sub.5 and C.sub.6 paraffinic hydrocarbons to branched chain, or isomerized, C.sub.5 and C.sub.6 paraffins which have a higher octane rating than the corresponding straight chain paraffins. One such method is disclosed in co-pending U.S. Ser. No. 08/732,823 entitled "A Method for Eliminating Carbon Oxides in Feeds To a C.sub.5 and C.sub.6 Paraffin Isomerization Process".
Such isomerization processes are well known to those skilled in the art as discussed in Chemical and Process Technology Encyclopedia, Douglas M. Considine, Ed., McGraw Hill Book Company, 1974, pp. 662-665. As discussed in this reference it is a common practice to also isomerize C.sub.4 hydrocarbons for use in alkylation processes and the like. One such method is disclosed in co-pending U.S. Ser. No. 08/732,828 entitled "A Method for Eliminating Carbon Oxides in the Hydrogen Feed to a Butane Isomerization Process". It is also noted that moisture must be minimized in the isomerization zone and that the amount of benzene in the paraffin feed stock should be minimized. It is also known to those skilled in the art that carbon oxides, even in small amounts, in the feed stream are extremely detrimental to the isomerization catalyst. Such carbon oxides are methanated over the isomerization catalyst. The methanation reaction produces water which permanently poisons the isomerization catalyst. Accordingly it has long been recognized that carbon oxides in the feed to the isomerization reactor must be minimized and desirably maintained at levels below 0.1 part per million by volume (ppmv). Various other refinery and chemical plant processes, such as processes catalyzed by organo metallic catalysts, such as alpha olefin polymerization processes and other processes catalyzed by catalysts which are sensitive to carbon oxides or water, require hydrogen containing very low amounts or no carbon oxides.
It has also been recognized that the presence of benzene and any other aromatics present in the paraffin feed to the isomerization reactor is detrimental since the benzene and other aromatics are hydrogenated over the isomerization catalyst causing an increase in reactor temperature which promotes unwanted cracking reactions and increased hydrogen consumption. Thus it is desirable to hydrogenate benzene and any other aromatics in the paraffin feed before the isomerization reaction zone in a unit such as a benzene saturation reactor.
Benzene saturation units have long been known to those skilled in the art and are used to saturate benzene and other aromatic compounds in such paraffinic streams. Such processes typically use a catalyst comprising from about 0.1 to about 1.0 weight percent platinum on a suitable catalyst support such as alumina or silica alumina. Such units typically operate at an inlet temperature from about 325 to about 800.degree. F. and a pressure from about 200 to about 700 pounds per square inch gauge (psig). Since carbon oxides temporarily poison the catalyst in the benzene saturation reactor it has been considered necessary to maintain the carbon oxide content of the streams charged to the benzene saturation reactor at low levels.
In recent years there has been increased interest in removing benzene and other aromatic components from C.sub.5 /C.sub.6 paraffinic streams and isomerizing C.sub.5 /C.sub.6 paraffinic streams because of the increased emphasis on the production of gasoline having a reduced aromatics content. Accordingly, improved methods have been sought for producing such gasolines from existing refinery streams.
Similar catalysts are used in aromatics saturation units for the treatment of other hydrocarbonaceous streams to reduce the amount of aromatics in such streams. Some such processes are solvent treating to saturate aromatics in naphtha streams, cyclohexane hydrogenation, lube oil hydrogenation and the like. The catalysts used in such units are frequently similar to those described above for the benzene saturation unit, although other catalysts known to the art for aromatics saturation may be used, and it has been considered necessary to maintain the carbon oxide content of the streams charged to the aromatics saturation reactor at low levels. The reaction conditions in such units are generally similar to the reaction conditions in the benzene saturation unit, except for lube oil hydrogenation which is typically at a higher pressure, and lower space velocity. Such hydrocarbonaceous streams may have a higher aromatics content than the paraffinic stream and the aromatics are typically present in the hydrocarbonaceous stream in minor amounts which may be up to about 30 volume percent or higher. The operation of such aromatics saturation units to saturate aromatics is well-known to those skilled in the art.
In many refineries the available hydrogen sources contain amounts of carbon monoxide, carbon dioxide or mixtures thereof up to as much as about 200 ppmv. Accordingly, in the past such hydrogen streams have been passed through a methanation reactor to react the carbon oxides to produce water and methane with the resulting water being removed prior to charging the hydrogen to isomerization reactors or other processes requiring carbon oxide-free hydrogen. Similarly, the carbon oxides and water have been removed prior to using such hydrogen streams in benzene saturation and aromatics saturation reactors.
Accordingly, an improved method has been sought for reducing the capital cost of such processes for producing hydrogen streams containing substantially no carbon oxides.